FIG. 4 is a schematic configuration view of a water heater including a downward combustion type-flat burner (3a) disposed on a lower surface of a burner body (40) in such a manner that a combustion surface (30a) is disposed facedown. In the water heater, the burner body (40) is provided in an upper region of the casing (4). Additionally, a sensible heat recovery type-primary heat exchanger (311) is disposed below the combustion surface (30a) of the flat burner (3a), and a latent heat recovery type-secondary heat exchanger (321) is disposed below the sensible heat recovery type-primary heat exchanger (311).
In such a water heater, all air taken in through an air supply pipe (43) by rotating a fan (not shown) in a fan case (41) is fed as primary combustion air to a mixing device (42). The air is then mixed with fuel gas fed through a gas supply pipe (44) in the mixing device (42) to generate a mixture gas. Further, the mixture gas is fed to the flat burner (3a) having the combustion surface (30a) in such a manner that the combustion surface (30a) is disposed facedown in the burner body (40).
In the above-described water heater, in order to minimize the height of the upper region of the casing (4), a blowout port (45) which opens laterally to one side of the fan case (41) is communicated with an inlet port (310) which laterally opens to one side surface of the burner body (40), as illustrated in FIG. 4. In this configuration, the burner body (40) and the fan case (41) are aligned in a lateral direction, which results in reducing the overall height of the casing (4).
However, the mixture gas is laterally blown from the blowout port (45) provided in the one side of the fan case (41) through the inlet port (310) provided in the one side surface of the burner body (40) into the burner body (40), whereby the mixture gas is laterally supplied to the combustion surface (30a) of the flat burner (3a). Therefore, distribution of the mixture gas over the combustion surface (30a) tends to become uneven due to such a blow direction. As a result, depending on a position of an ignition electrode, there are problems that not only ignition performance degrades but combustion performance deteriorates because the mixture gas unevenly combusts over the combustion surface (30a). In view of the ignition performance, it is considered that the ignition electrode could insert through another side surface of the burner body (40) away from the blowout port (45). However, such an ignition electrode can not be attached through a frontward portion where a door opening/closing an opening of the casing (4) is provided, which results in difficulty in assembly and maintenance.
In view of the combustion performance, although not shown, a burner body having a mixture gas inlet port opening upward in a central portion of an upper surface and a fan case having a blowout port opening downward so as to be communicated with the mixture gas inlet port are used, so that an assembly structure in which the fan case stands on the upper surface of the burner body could be designed. Such an assembly structure facilitates simultaneous spread of the mixture gas over the entire interior of the burner body, which advantageously results in even distribution of the mixture gas over the combustion surface, whereas disadvantageously results in simultaneous filling of the entire interior of the burner body with the mixture gas. As a result, when spark discharge from the ignition electrode is delayed, the ignition is generated in a space filled with the mixture gas, which may cause an explosion ignition. Furthermore, according to the assembly structure described above, since the fan case accommodating a fan stands on the burner body, the overall height of the casing increases, which results in a larger water heater in size.